1. Field of the Invention
The present invention pertains to electronic amplifiers, and, in particular, to an amplifier that may be used in, for example and without limitation, a transmitter for a railroad track circuit, wherein certain amplifier functionality is implemented in a configurable integrated circuit device, such as a field programmable gate array (FPGA) or a complex programmable logic devices (CPLD).
2. Description of the Related Art
In rail signaling systems, it is often necessary to transmit several different signal types, each required for a different function, on a RR track circuit. For example, in many rail signaling systems, the following three signals are transmitted on the track circuit: (i) a modulated train detection signal that is used to detect the presence of a train in a track circuit block, (ii) a modulated cab signal that is transmitted to the train in order to communicate track status, speed command and conditions to the crew, and (iii) a modulated TWC (track warrant control) signal. Traditionally, the train detection signal and the cab signal may have used, for example and without limitation, an ASK modulation scheme, FSK modulation scheme, an Impulse modulation scheme, or some other proprietary or non-proprietary modulation scheme, and the TWC signal was an FSK modulated signal. However, more recently, binary frequency shift keying (BFSK) digital train detection incorporating track ID, which employs a BFSK modulated train detection signal, is becoming widely accepted as being much more secure than legacy modulated carrier detection. There is thus a desire to migrate/upgrade to such systems where possible.
Thus, as the signaling equipment of many transit authorities becomes obsolete, a large market opportunity exists for replacing legacy equipment with new technology that uses the secure BFSK modulated digital train detection just described or some other new and/or different modulation scheme. However, it is typically only feasible to replace equipment incrementally, and therefore a real technical challenge arises as to how to transmit and decode digital secure train detection signals while simultaneously transmitting a distinct and different legacy cab signal and/or legacy TWC signal such that the train detection signal equipment can be upgraded first while still supporting the legacy cab and TWC signals until the cab is upgraded at a later time. By this method, transit authorities can slowly migrate to a new platform by focusing on updating ground equipment first, and converting cab equipment at a later time. This is only possible, however, if multiple signals (train detection, cab signaling and TWC signaling) can be simultaneously and independently transmitted over a single twisted pair of wire.
One way of doing so using prior art technology would involve the use of a transmitter employing three separate Class AB linear amplifiers, one for each signal. However, as is known in the art, a Class AB amplifier is relatively large and has a maximum theoretical efficiency of 78% (in practice, approximately 50% is typical with low distortion). Thus, a solution employing multiple separate Class AB linear amplifiers as just described would not be efficient and would have a very large physical size.
There is thus a need for an amplifier solution wherein multiple signals (e.g., train detection, cab signaling and TWC signaling) can be simultaneously and independently transmitted over a single twisted pair of wire that is both efficient and minimally sized.